Healthy fats

The science

The Nurses’ Study found that eating less trans fat can improve fertility, and adding in healthy unsaturated fats can boost it even further.

The foods we eat contain four main types of fat: saturated, monounsaturated, polyunsaturated and trans fat. Each one is made from a string of carbon atoms bonded to hydrogen atoms. What makes one fat different from another is the length and geometry of its carbon chain and how many hydrogen atoms it sports. Each fat family has its own characteristics and effects on health. Three of these – saturated, monounsaturated and polyunsaturated – should be part of every diet. Trans fat however, is something you can, and should, avoid.

Monounsaturated fats should be emphasised in your diet, and can be found in olives and olive oils, canola, peanut and other nut oils, nuts, nut butters, avocados, sesame, pumpkin and other seeds. You should also emphasise poly-unsaturated fats, which can be found in vegetable oils, especially corn, soybean and safflower oils, soybeans and other legumes, walnuts, fatty fish such as tuna, salmon, herring and anchovies.

Saturated fats can be found in red meat, whole milk, cream, butter, cheese and ice cream, coconuts and coconut products, palm oil – and you should aim to eat less of these fats.

Trans fat can be found in any product containing partially hydrogenated vegetable oil. This includes many solid margarines, vegetable shortening, most commercial baked goods, and most fast foods (many products that once contained trans fats are now available in trans-free forms). You should aim to avoid trans fat completely.   

The Nurses’ Study found that the more trans fat in the diet, the greater the likelihood of developing ovulatory infertility. Why does this happen? Trans fat impacts the activation of PPAR-gamma which means that the body becomes more resistant to insulin. Just 6 grams a day of trans fat cuts PPAR gamma activity in half. This in turn means higher blood sugar and insulin levels which reduce fertility. At the same time, trans fat increase inflammation throughout the body, which interferes with ovulation, conception and early embryonic development.

In contrast, unsaturated fats actually sensitise muscle and other tissues to insulin’s “open up for sugar” signal. Linoleic acid, the most common polyunsaturated fat, activates the PPAR-gamma.

In studies, healthy people given unsaturated fats instead of saturated or trans fats develop greater sensitivity to insulin. The same thing happens in people with type 2 diabetes or those who are overweight. Other experiments show that eating unsaturated fats in place of saturated or trans fats decreases the body’s production of glucose and appears to calm inflammation. In short, unsaturated fats do things to improve fertility – increase insulin sensitivity and cool inflammation – that are the opposite of what trans fat do.

Think of trans fats as the evil cousins to the healthy omega-3 fats in fish, flaxseeds and walnuts. The only natural sources of trans fats are bacteria living in the stomachs of cows, sheep, deer and other ruminants. Until a century ago, they were the only source of it. But in the nineteenth century chemists discovered they could turn a liquid vegetable oil into a solid or semisolid by heating it and bubbling hydrogen gas through it. Oils  that have undergone this process, known as partial hydrogenation, don’t spoil or turn rancid as readily as non-hydrogenated oils. They can be heated to high temperatures again and again in restaurant deep fryers without breaking down. These characteristics have made them very popular. It’s estimated that 95% of prepared cookies, 100% of crackers and 80% of frozen breakfast products contain partially hydrogenated fats. That means they also contain trans fat, an inescapable by-product of partial hydrogenation.

Trans fats are also not good for your heart. They boost LDL, especially the small, dense, LDL particles that are most damaging to arteries. It depresses HDL which protects arteries. Trans fats also make blood platelets stickier than usual and so are more likely to form artery blocking clots.

Omega-3 fats, such as those found in fish, are needed for brain development before and after birth. A large, long-term study of almost 12,000 women in the UK found that children of women who ate less than twelve ounces of fish a week – were more likely to score in the lowest quarter on verbal IQ tests. They were also more likely to have problems with fine motor control, communication and scores on social development tests (Hibbeln et al, Lancet 2007). Omega 3 rich fish include salmon, herring, mackerel and sardines. A good weekly target it to eat fish two to three times a week.

Your change for the week

  • Cut back on trans fat & add in mono-polyunsaturated fats
  • Food manufacturers are now required to list trans fats. Take a look at your food labels to understand whether the food you are eating contains trans fats.

Here are some typical trans fat content of foods produced/prepared with trans fats:

  • French fries – 4.7 – 6.1g
  • Chicken nuggets – 5g
  • Enchilada – 2.1g
  • Burrito – 1.1g
  • Pizza – 1.1g
  • Tortilla chips – 1.6g
  • Popcorn – 1.2g
  • Granola bar – 1g
  • Pie – 3.9g
  • Danish – 3.3g
  • Doughnuts – 2.7g
  • Cookies – 1.8g
  • Cake – 1.7g
  • Brownie – 1g
  • Muffin – 0.7g
  • Vegetable shortening – 2.7
  • Hard margarine (stick) – 0.9-2.5g
  • Soft margarine (tub) – 0.3-1.4g
  • Pancakes – 3.1g
  • Crackers – 2.1g

Recipe inspiration

Banana and peanut butter smoothie
(Peanut butter is a good source of monosaturated fat)

2 tbsp peanut butter, 1 tsp honey, ½ cup of milk, ¾ cup of natural yoghurt, 1 banana

With a hand mixer or blender, blend the milk, yogurt, banana and peanut butter.

Sesame salmon, purple sprouting broccoli & sweet potato mash
(Fatty fish like salmon are rich in omega-3 fatty acids, fats that are good for the heart and brain.)

  • 1 ½ tbsp sesame oil
  • 1 tbsp soy sauce
  • Thumb sized piece of ginger, grated
  • 1 garlic clove, crushed
  • 1 tsp honey
  • 2 sweet potatoes, scrubbed and cut into wedges
  • 1 lime, cut into wedges
  • 2 salmon fillets
  • 250g purple sprouting broccoli/ tenderstem broccoli
  • 1 tbsp sesame seeds
  • 1 red chilli, thinly sliced (de-seeded if you don’t like it too hot)

Heat oven to 200C/180 fan/ gas 6 and line a baking tray with parchment. Mix together 1/2 tbsp sesame oil, the soy, ginger, garlic and honey. Put the sweet potato wedges, skin and all, into a glass bowl with the lime wedges. Cover with cling film and microwave on high for 12-14 mins until completely soft. Meanwhile, spread the broccoli and salmon out on the baking tray. Spoon over the marinade and season. Roast in the oven for 10-12 mins, then sprinkle over the sesame seeds. Remove the lime wedges and roughly mash the sweet potato using a fork. Mix in the remaining sesame oil, the chilli and some seasoning. Divide between plates, along with the salmon and broccoli.